Union types are my biggest wish for Dart, but unfortunately it doesn't look like they'll be added any time soon.

They've recently added support for compiler-enforced pattern matching over sealed classes, which I suppose does get you halfway there though.

What languages could fit that description today? I don't really understand what it even means but maybe I could understand better if I could look at examples in languages which have them.

For example Rust: https://doc.rust-lang.org/std/keyword.enum.html

Also Scala

Swift, Typescript depending on your opinion with structural typing, and Rust.

Think Scala, Elm and Haskell have it as well.

Having that and elixirs pattern matching would be insane.